1. Field of the Invention
The present invention relates to a flow control valve which controls a supply flow quantity of fluid to an actuator in accordance with movement of a spool therein.
2. The Related Art of the Invention
There is known this kind of a flow control valve described in Japanese Unexamined Patent Publication No. 8-74749. FIG. 6 shows the flow control valve in which a spool bore 4 is formed in a valve body 1 and a spool 6 is slidably incorporated in the spool bore 4.
A pump port 2, as well as a supply port 3 are positioned in a way so as to be connected to the spool bore 4. The pump port 2 is connected to a discharge side of a vane pump (not shown) and the supply port 3 is connected to an actuator (not shown).
A set spring 5 is pressed against an end of the spool 6, which is displaced by balance of this spring force and a pump discharge pressure supplied from the pump port 2.
A control rod portion 7 is disposed coaxially with the spool 6 therein, extending into a side of the supply port 3, and inserted into a communicating hole 8 of a partition wall formed in the spool bore 4, thus providing a main orifice 9 by a clearance between the control rod portion 7 and the communicating hole 8. An opening of the main orifice 9 varies with the movement of the spool 6 and a flow quantity of fluid introduced in the supply port 3 is determined in accordance with this main orifice opening.
Further, a return port 12 is connected to the spool bore 4 and a reservoir (not shown). A land portion 11 is formed in the outer periphery of the spool 6 and a circular groove 10 is formed adjacent the land portion 11.
When the spool 6 is positioned in such a way that the land portion 11 closes the return port 12, an entire quantity of the fluid sent from the pump port 2 flows through the main orifice 9 to the supply port 3, and on the other hand, when the land portion 11 opens a part of the return port 12, a part of the fluid is returned back from the return port 12 to the reservoir.
FIG. 7 shows flow characteristics of a pump discharge quantity of the fluid controlled by the flow control valve. The discharge quantity of the pump (not shown) increases in proportion to a pump rotation speed, and the flow control valve is adapted to control a flow quantity of the fluid supplied to the supply port 3 to be constant after the pump rotation speed reaches a predetermined value.
When an increase in the rotation speed N of the pump causes the discharge quantity Q of the pump to increase, a flow resistance of the fluid is also increased when the fluid flows from the pump port 2 through the main orifice 9 to the supply port 3, thereby increasing a pressure upstream of the main orifice 9. This allows the pressure acting on the end face of the spool 6 to move the spool 6 in the direction shown in an arrow Y against the set spring 5. The movement of the spool 6 causes the land portion 11 to open the return port 12. As a result, a part of a discharge quantity of the fluid from the pump port 2 is returned back from the return port 12 to the reservoir as an extra flow quantity, thus controlling the flow quantity of the fluid to the supply port 3.
An increase in pressures upstream of the main orifice 9 causes a movement amount of the spool 6 to increase, whereby an opening of the return port 12 is also increased, thus increasing the extra flow quantity of the fluid to be back to the reservoir.
Therefore, a supply flow quantity of the fluid passing through the main orifice 9, as shown in FIG. 7, is adjusted to be a preset value in accordance with rotation speeds of the pump. Herein FIG. 7, as an example, shows the characteristics that the supply flow quantity is constant regardless of the rotation speeds of the pump.
A concave portion 13 is formed in a valve body 1 at a position in the direction extending from the return port 12 in such a way that the concave portion 13 is opened to the spool bore 4.
The concave portion 13 has a diameter having the same dimension as a diameter of the return port 12 and is processed together with the process of the return port 12 by a drill used in forming the return port 12. Therefore, a cone face 13a is formed in a center of the concave portion 13, corresponding to the cutting edge of the drill.
A width L1 of the concave portion 13 in the axis line of the spool 6 is formed greater than a width L2 of the land portion 11 in the axis line of the spool 6. Note that a width L3 of the return port 12 in the axis line of the spool 6 is equal to the width L1 of the concave portion 13.
The reason why in the flow control valve the circular groove 10 is thus disposed in the spool 6 and the concave portion 13 is positioned facing the return port 12, and the width L1 of the concave portion 13 is greater than the width L2 of the land portion 11 is as follows.
The first reason is to improve a pressure balance in the circumferential direction of the spool 6. Opening the return port 12 allows a part of the operating fluid from the pump port 2 to flow into the return port 12 as shown in “f1” of FIG. 6. Then, a pump discharge pressure or a pressure close to it is applied on a side face of the spool 6 facing the return port 12. This situation leads to a state where, in regard to the spool 6, an offset load is applied to the spool 6 in one direction of the circumferential face, thereby deteriorating the balance of the spool 6. As the concave portion 13, however, is disposed as described above, the flow “f2” passing through the concave portion 13 other than the flow “f1” occurs. As a result, the pressure the same as the above-mentioned pressure is applied in the opposing direction on the outer periphery of the spool 6, whereby the operating forces generated by these fluid forces are cancelled out with each other, improving the balance of the forces in the circumferential direction of the spool 6.
The second reason is to reduce friction acting on the spool 6.
As explained in the first reason, application of the offset load to the spool 6 causes the friction between the spool 6 and the spool bore 4 to increase corresponding to the offset load. Such occurrence of the friction leads to deterioration of the axial movement of the spool 6. In case the spool 6 does not move in the axial direction smoothly, a spring force of the set spring 5 is required to increase when the spool 6 moves in the right direction in FIG. 6, namely in the returning direction.
An increase in the spring force of the set spring 5, however, causes the pump inner pressure to be highly maintained, resulting in an increase in energy losses. Accordingly, an increase in the spring force of the set spring 5 is in fact limited to a certain degree. In order to solve such problem, the circular groove 10 is disposed adjacent the land portion 11, thereby reducing a contact area between the spool bore 4 and the spool 6. And the width L1 of the concave portion 13 is greater than the width L2 of the land portion 11, whereby when the return port 12 is opened, the flow “f2” passing through the concave portion 13 is ensured. Accordingly the offset load exerting on the spool 6 is reduced, thus reducing the friction.